Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

[Image: see text] Extensive research has been devoted to the development of thermally activated delayed fluorescence emitters, especially those showing pure-blue emission for use in lighting and full-color display applications. Toward that goal, herein we report a novel weak donor, 1,4-azaborine (AZB), with complementary electronic and structural properties compared to the widely used dimethylacridan (DMAC) or carbazole (Cz) donors. Coupled with a triazine acceptor, AZB-Ph-TRZ is the direct structural analogue of the high-performance and well-studied green TADF emitter DMAC-TRZ and has ΔE(ST) = 0.39 eV, a photoluminescence quantum yield (Φ(PL)) of 27%, and λ(PL) = 415 nm in 10 wt % doped mCP films. The shortened analogue AZB-TRZ possesses red-shifted emission with a reduced singlet–triplet gap (ΔE(ST) = 0.01 eV) and fast reverse intersystem crossing (k(RISC) of 5 × 10(6) s(–1)) in mCP. Despite a moderate Φ(PL) of 34%, OLEDs with AZB-TRZ in mCP showed sky-blue emission with CIE(1931)(x,y) of (0.22,0.39) and a maximum external quantum efficiency (EQE(max)) of 10.5%. Expanding the chemist’s toolkit for the design of blue donor–acceptor TADF materials will enable yet further advances in the future, as AZB is paired with a wider range of acceptor groups.